Significance Our previous studies have established that neutrophil emigration into the airway tree closely correlates with the focal epithelial necrosis resulting from ozone exposure in rhesus monkeys. Objectives We are currently testing the hypothesis that neutrophils play a beneficial role in the repair of ozone-injured bronchiolar epithelium by removing necrotic epithelial cells in the early stages of the repair process. Results Rhesus monkeys have been given a functional blocking monoclonal antibody to CD18 by the cephalic main and exposed to ozone or filtered air for an eight hour period. Monkeys given the anti-CD18 antibody and exposed to ozone showed significant inhibition of neutrophil emigration and epithelial repair along with accumulation of necrotic epithelial cells at 24 and 48 hours after ozone exposure, when compared to ozone-exposed monkeys without the antibody. Two additional groups of monkeys, one given the antibody and the other the isotype control antibody, were exposed to ozone and then complement 5a was instilled via a pediatric bronchoscope into the right middle and right caudal lobes at the end of exposure. Using a new state-of-the-art assay for detecting dead cells which relies on detecting loss of membrane integrity by the incorporation of an impermeant dye into the nucleus, the distribution of necrotic cells was characterized throughout the airway tree. In the exposed anim als in which neutrophil migration was blocked by the antibody, the lung lobes which received complement 5a were entirely free of dead cells and the epithelial surface was partially recovered within four hours of exposure cessation. This was not the case in the portions of the lungs which did not receive C5a. Neutrophils removed most of the dead cells in animals without the antibody. Future Directions We are continuing these studies to establish whether proliferation, migration or differentiation phases of the repair process are enhanced or down regulated by the presence of migrating neutrophils. KEYWORDS neutrophil, bronchiole, ozone, wound repair FUNDING NIH Grant ES00628